FIG. 1 shows a basic current-steering DAC where a plurality of current sources 101 is connected to analog output nodes 102, 103 via a plurality of switch pairs 104. The switch pairs 104 are controlled by the respective control signals 105 which are generated by the control signal generation circuits 106. The control signal generation circuits 106 receive a clock signal 107 and a digital input word 108 which represents a desired analog output of the DAC. For actual DACs, the control signals 105 will be coupled to the analog output nodes 102, 103 through the switch pairs 104.
One approach to reduce the coupling noise is to return the analog output nodes to “zero” by directly shorting the analog nodes to “zero”. For the DAC without using return-to-zero (RTZ) techniques, the output signal power of the DAC falls off at a rate given by sin(x)/x as shown in FIG. 2. For the DAC using return-to-zero techniques, the output signal power of the DAC falls off at a rate given by sin(nx)/nx as shown in FIG. 3. However, this approach has disadvantages. The coupling noise of control signal switching to the output nodes are only attenuated not isolated, such that the effective impedance of the RTZ transistor strongly determines the attenuation of the coupling noise. To achieve large attenuation, the size of the RTZ transistor should be large. In addition, the RTZ control signals will be coupled to the output nodes via the parasitic capacitances.
Another approach to reduce the coupling noise is the use of isolation transistors. However, the settling time of the analog output nodes is increased, due to the use of isolation transistors, which is innegligible at high operation speed. The slow discharging action of the internal nodes between the switches and the isolation transistors causes different rise and fall times and large settling times. To solve this problem, the invention provides the internal nodes extra discharging paths and also returns the analog output nodes to “zero”.